I. Field of the Invention
The field of the invention is that of the motor industry. More specifically, the invention relates to subframes supporting the engines of motor vehicles or rear ends.
II. Description of Related Art
In a commonly used method, an engine subframe comprises two shells made from pressed sheet metal and joined together, by welding for example.
One of the functions of this subframe is to withstand the forces generated by the engine torque during the acceleration or deceleration of the vehicle.
It should be noted, with reference to FIG. 1, that a subframe of this kind is fixed to the rear part of the vehicle body. A shock absorber 2 or “add-on” which is independent of the subframe and has no contact with it, except in case of impact, is placed in the extension of the subframe.
Such a subframe is also coupled to lower arms 3 supporting the wheel or carrying wheel support means 31 (shown schematically), an anti-roll bar 32 connecting these arms and being held on the subframe. Such an engine subframe is therefore an intermediate structure between the body and the steering members of the front end.
A subframe of this type terminates at its forward end approximately at the position of the wheel axle, and is consequently known as a “short subframe”, by contrast with a full subframe which extends to the front end cross-member behind the fender.
These subframes facilitate preparation for assembly in the bodywork assembly plant, since they can be prepared off-line, with the steering, the anti-roll bar, the torque link, and the wishbones being assembled before the final coupling to the body.
A front end assembly made in this way is very compact.
In operation, it must allow the free travel of the surrounding movable elements such as the suspension arms (points A, B and E), the tie rods, and the lateral transmissions of the anti-roll bar.
The impact energy absorption function of short subframes imposes two significant design constraints on these members.
Firstly, the short subframe must withstand forces originating from the deformable front structure (add-on) in a frontal impact, these forces being primarily oriented in the direction X of the vehicle. The force path will therefore require shapes and sections designed to “absorb” its planned crushing.
At the same time, the suspension arm is required to have a failure resistance such that it becomes fusible only above a level of loading dictated by clearly defined safety criteria. Consequently, it must not have excessively thin cross sections, but it is also impossible to form through holes in its surface, particularly in line with the front side member, for access for screwing the subframe to the body in the body assembly plant.
The frame is also connected to the body in its rear part by resilient blocks which are housed in enclosures under the floor.
In its front part, in view of the aforementioned context and constraints, the fixing can only be made to the rear of the joint A of the arm on the subframe (such an arm is shown in FIGS. 2 and 3).
Furthermore, manufacturers' specifications require that this fixing should be carried out by means of a filtration block located immediately below the side member.
The front fixing is therefore considerably offset in the direction Z from the horizontal plane (approximately 250 mm from the area of point A).
In this space, particularly on the left-hand side, the protrusion of the “nose” 41 of the gearbox 4 makes it particularly difficult to design the front fixing of the transmission to conform to the specifications concerning rigidity. It should be noted that current specifications require a rigidity Ky of 100 daN/mm at the point identified by 32. This value of rigidity makes it necessary to design, in a very confined space, a vertical fixing of the block 33 which is very “floating”, but sufficiently rigid to meet the specifications.
Different topologies for the implementation of the fixing of the short subframe to the front side members have been proposed in the prior art.
According to a first known method, the connection is provided by an articulated link, equipped with resilient joints at each end, connecting the subframe to the side member. This solution is used in an automobile such as the RENAULT MEGANE (registered trademark).
This solution complicates the mounting of the assembly, since there are numerous assembly points (upper and lower).
Furthermore, a link positioned vertically and pivoted at each end cannot develop any transverse rigidity and therefore does not meet the aforementioned specification (100 daN/mm).
Another solution, shown in FIGS. 2 and 3, is to divide the path to be covered between the side member 5 and the subframe 1 into two parts. A first structure 331, welded to the side member 5, extends down to mid-height, while another structure 332 rises from the subframe 1 to meet it. This solution is used by automobile manufacturers such as MITSUBISHI and VOLVO (registered trademarks).
The fixing between the two elements is provided by rigid connection (screwing) or by resilient blocks. In this case also, the connection is not made at the correct height, since no “downward extending” structure is permitted, and all the vertical offsetting must be provided from the subframe.
There are also known solutions using a structure welded to the subframe, also called a “horn”, made by pressing or from bent tubing and taking up the offset in the direction Z, and especially in the direction Y, between the lower part of the subframe and the upper part for connection to the side member. This solution is used by an automobile manufacturer such as BMW MINI and on an automobile such as the FORD FIESTA (registered trademarks).
This offset in the direction Y is not possible in some applications, particularly when it gives rise to interference with the gearbox in a section X.